On Average, Winter's Coldest Is Colder Than You Might Think

It's January, and for many areas of the country this is the time of year when temperatures are typically at their lowest.

If you've been following weather reports for long, you're probably very familiar with the concept of the day's "normal" low or "average" low temperature. For any given location, the normal low temperature for a given date is a figure based on the average of all the lows on that date over a 30-year period. In the United States, normal temperatures are computed by the National Oceanic and Atmospheric Administration (NOAA) using 1981 through 2010 as the 30-year base period.

Though it's sometimes referred to as an "average" low, the normal low for a given day is not strictly an average. NOAA uses various techniques to account for missing historical data and to smooth out the small day-to-day ups and downs in the strict mathematical averages.

We routinely compare recent, current and forecast high and low temperatures to the daily normal temperatures in order to give a sense of how the weather compares to the typical conditions for a given place and time. Those normals are also widely used by people to make decisions about when and where to vacation and even when or where to do business.

But if you only look at the daily normal (or daily "average") temperatures for a place, it turns out you're not getting the entire picture.

With very few exceptions, every winter brings a spell of below-average daily low temperatures no matter where you are in the U.S. And it turns out that when you look at annual low temperatures – the lowest temperature each year, rather than each day – the averages look very, very different.

Using NOAA's climate database, we computed the average annual low temperature for all of the first-order climate sites in the U.S., along with several other important locations. In order to capture the low temperature winter by winter, we used 12-month periods running from July 1 through June 30, rather than the conventional calendar year. We used a 30-year period beginning July 1, 1980, and ending June 30, 2010, to mirror NOAA's base period as closely as possible.

The results were in some cases quite surprising. Take, for instance, Orlando.

It would be entirely reasonable to plan a winter vacation to that city's famous theme parks, given that the daily average low goes no lower than 49 during the heart of winter in January.

But you might be as shocked as we were to learn that in an average winter Orlando's coldest night brings a low of 29 degrees. And that's just the average; in the base period we used, the annual low ranged from 37 in the El Niño winter of 1997-98 to a frigid 19 in the arctic outbreak of January 1985.

Needless to say, if you just looked at the chilliest daily normal low of 49 (and for some reason failed to check the latest forecast), you might be a bit stunned if you found yourself in Orlando on its coldest night of any given winter. Even in the mildest of Orlando winters, you can count on at least one night to go more than 10 degrees below the daily normal low.

The slideshow above shows similar comparisons for 30 major metropolitan areas in the United States. Below, we have regional maps showing the average annual low temperature for even more locations across the country.

In the Northeast, an average winter sees temperatures fall into the single digits in the coastal cities and below zero over much of the interior at least once.

Even in the Deep South most places can expect lows to reach the teens at least once in an average winter.

Despite the warm waters of the Gulf of Mexico, most places along its shores can expect a hard freeze in an average winter. Only in South Florida and the Florida Keys do temperatures typically stay above freezing every night of every year.

Temperatures in the double digits below zero are relatively common in western parts of the Midwest. Farther east, areas downwind of parts of the Great Lakes tend not to get quite as cold due to the moderating effect of the lakes on arriving cold air masses. Additionally, cold air passing over the lakes often creates cloud cover that limits nighttime cooling downwind.

With few topographical influences, the average winter's lowest temperature in the High Plains and Central Plains is largely a function of latitude, which influences the angle and amount of sunlight received in the winter months.

The low deserts of Southern California and Arizona are among the few places in the Lower 48 that avoid the freezing mark most years. By contrast, the high mountain valleys of Colorado routinely feature extremely low temperatures on an annual basis.

Prevailing westerly winds off the Pacific Ocean and the high terrain of the Cascades both serve to limit the ability of arctic air to intrude into the Pacific Northwest. As a result, cities west of the Cascades have annual low temperatures that compare favorably with cities in the Deep South despite having a cooler climate overall.